I am afraid it will be required to create separate shipping methods for each vendor
The way it seems to work is:
1) Admin creates shipping methods but does not apply them to vendor
2) Vendor creates new shipping method. In the "Shipping Charges" tab, vendor has access to admin's methods, and can adapt them to create new ones for themselves.
3) Vendor's newly adapted methods appear at checkout.
The only problem I've seen so far is that if the vendor comes back later and tries to create another shipping method, the previous ones don't have their values repopulated and revert to zero. So, if the user clicks "Save" after creating new method, it saves the other ones with zero value. Looks like a bug to me.